The present invention relates to a pneumatic tire, more particularly to an improvement in the bead structure which is capable of improving the bead durability and suitable for use in a heavy duty tire such as a truck/bus tire.
A pneumatic tire is provided in each bead portion with a bead core whose cross sectional shape is a circle, rectangle, parallelogram, hexagon or the like.
In a conventional pneumatic tire for heavy duty use such as a truck/bus tire, the cross sectional shape of the bead core is usually a compressed hexagon such as shown in FIG. 19. The bead core (b) is located such that the longer side G1 is substantially parallel to the bead seat j1 of the rim J when mounted on the rim which is typically a standard 15 degree drop-center rim. Around the bead core (b), a carcass ply (c) is turned up to be secured thereto.
When such tire is vulcanized, the topping rubber of the carcass ply (c), the topping rubber of the bead wire, the topping rubber of a bead wrapping cambus (bw) around the bead core (b) and the like partially migrate or flow and collect between the longer side G1 of the bead core (b) and the carcass ply (c) to form a rubber layer (A).
It is difficult to estimate the thickness of the collected rubber layer (A) because it varies depending on various parameters such as the carcass cord tension during tire vulcanization, variation of the carcass turnup profile, variation of the topping rubber volume and the like. Further, the thickness of the collected rubber layer varies not only between different tires but also between the different positions in the same tire.
If the thickness of the gathered rubber layer increases, the thickness of rubber (chafer rubber) under the carcass is decreased, and accordingly the contact pressure between the bead and rim's bead seat decreases because the collected rubber is mainly topping rubber which is softer than chafer rubber. As a result, the contact pressure between the axially outer surface of the bead portion and the axially inner surface of the rim flange j2 is relatively increased, and the carcass ply turnup portion (f) tends to be damaged, which greatly decreases the bead durability.
This problem is particularly remarkable in a heavy duty pneumatic tire which is mounted on a tapered rim having a relatively large taper angle and used under heavy load and high pressure conditions.
If the thickness of the rubber under the carcass is increased in order to lessen the influence of the variation of the collected rubber layer thickness, the rim mounting operation becomes very difficult.